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Biomedical subjects

U Hopfer

Publications and source records attributed to U Hopfer.

At least 109 records · Page 6Linked to original sources
PubMed1978

Small intestinal sugar and amino acid transport in semistarvation.

The effect of semistarvation on small intestinal transport of D-glucose, L-valine, and NaCl was studied in an in vitro system of isolated rat brush border membrane vesicles. Whereas semistarvation enhanced the transport rate for L-valine by 19-29%, there was no change in D-glucose transport. When energy in the form of a NaSCN gradient was supplied to the membrane vesicles prepared from semistarved animals, L-valine was concentrated to a greater extent than those from well-fed animals. Strain differences were observed in the manner semistarvation affected NaCl transport across the brush border membrane. Semistarvation increased the NaCl transport rate by a factor of 3.5 in one rat strain and not at all in another. These results provide a partial explanation for the cellular basis of elevated neutral amino acid absorption by the small intestine in semistarvation.

Amino Acids↗
PubMed1977

Kinetics of Na+-dependent D-glucose transport.

The kinetic parameters of the Na+-dependent glucose transport system have been determined in isolated membrane vesicles for D-glucose, Na+, and phlorhizin. The D-glucose flux measurements were carried out by the equilibrium exchange procedure at constant external and internal Na+ concentrations and zero potential. Equations were developed to extract information about Km and Vmax from uptake measurements into a vesicle population that is heterogeneous with respect to size (surface to volume ratio). The Km for D-glucose was 14 mM and independent of the Na+-concentration, while the Vmax was strongly Na+-dependent and increased 15-fold between 1 and 100 mM Na+. The Km of Na+ for activation of the Vmax was 18 mM. The calculated KI values for phlorhizin were 2.7 and 1.9 micrometer when determined under active and equilibrating D-glucose flux conditions, respectively.

Animals↗
PubMed1977

Isolated membrane vesicles as tools for analysis of epithelial transport.

In recent years a methodology has been developed to use vesicles of the isolated brush border and basolateral plasma membranes for intestinal transport studies. The methodology and information gained with the vesicle systems are discussed using the examples of nonelectrolyte transport. In particular, results are presented on the mechanisms of D-glucose and neutral amino acid translocation across both plasma membranes and the coupling of Na+ with sugar and amino acid transport in the brush-border membrane. Furthermore, the kinetic parameters of the Na+-dependent glucose transport system, determined with an equilibrium exchange procedure, and the effects of semistarvation on the transport properties of the brush border membrane are described.

Amino Acids↗
PubMed1976

The surface membrane of the small intestinal epithelial cell. I. Localization of adenyl cyclase.

The subcellular distribution of adenyl cyclase was investigated in small intestinal epithelial cells. Enterocytes were isolated, disrupted and the resulting membranes fractionated by differential and sucrose gradient centrifugation. Separation of luminal (brush border) and contra-luminal (basolateral) plasma membrane was achieved on a discontinuous sucrose gradient. The activity of adenyl cyclase was followed during fractionation in relation to other enzymes, notably those considered as markers for luminal and contraluminal plasma membrane. The luminal membrane was identified by the membrane-bound enzymes sucrase and alkaline phosphatase and the basolateral region by (Na+ + K+)-ATPase. Enrichment of the former two enzymes in purified luminal plasma membrane was 8-fold over cells and that of (Na+ + K+)-ATPase in purified bisolateral plasma membranes was 13-fold. F--activated adenyl cyclase co-purified with (Na+ + K+)-ATPase, suggesting a common localization on the plasma membrane. The distribution of K+-stimulated phosphatase and 5'-nucleotidase also followed (Na+ + K+)-ATPase during fractionation.

Adenosine Triphosphatases↗
PubMed1976

Sodium/proton antiport in brush-border-membrane vesicles isolated from rat small intestine and kidney.

Studies on proton and Na+ transport by isolated intestinal and renal brush-border-membrane vesicles were carried out to test for the presence of an Na+/H+-exchange system. Proton transport was evaluated as proton transfer from the intravesicular space to the incubation medium by monitoring pH changes in the membrane suspension induced by sudden addition of cations. Na+ transport was determined as Na+ uptake into the vesicles by filtration technique. A sudden addition of sodium salts (but not choline) to the membrane suspension provokes an acidification of the incubation medium which is abolished by the addition of 0.5% Triton X-100. Pretreatment of the membranes with Triton X-100 prevents the acidification. The acidification is also not observed if the [K+] and proton conductance of the membranes have been increased by the simultaneous addition of valinomycin and carbonyl cyanide p-trifluoromethoxyphenylhydrazone to the K+-rich incubation medium. Either valinomycin or carbonyl cyanide p-trifluoromethoxyphenylhydrazone when added alone do not alter the response of the membranes to the addition of Na+. Na+ uptake by brush-border microvilli is enhanced in the presence of a proton gradient directed from the intravesicular space to the incubation medium. Under these conditions a transient accumulation of Na+ inside the vesicles is observed. It is concluded that intestinal and renal brush-border membranes contain a NA+/H+ antiport system which catalyses an electroneutral exchange of Na+ against protons and consequently can produce a proton gradient in the presence of a concentration difference for Na+. This system might be involved in the active proton secretion of the small intestine and the proximal tubule of the kidney.

Animals↗
PubMed1976

Jejunal and ileal D-glucose transport in isolated brush border membranes.

D-Glucose transport was investigated in isolated brush border membranes from small intestine. The transport properties of membranes from upper jejunum were compared with those from terminal ileum. The jejunal membranes accumulate D-glucose to a greater extent than the ileal membranes when supplied with energy in the form of a NaSCN gradient. This difference in behavior is similar to that of the more intact epithelial preparations and suggests that the isolated membranes actually reflect the state present in intact cells. Ileal membranes transported D-glucose about two to three times slower than the jejunal ones, which can partially explain the lower sugar accumulation.

Animals↗
PubMed1976

Differences in neutral amino acid and glucose transport between brush border and basolateral plasma membrane of intestinal epithelial cells.

A comparison of L-valine and D-glucose transport was carried out with vesicles of plasma membrane isolated either from the luminal (brush border) or from the contra-luminal (basolateral) region of small intestinal epithelial cells. The existence of transport systems for both non-electrolytes was demonstrated by stereospecificity and saturability of uptake, as well as tracer coupling. Transport of L-valine and D-glucose differs markedly in the two types of plasma membrane with respect to stimulation by Na+. The presence of Na+ stimulated initial L-valine and D-glucose uptake in brush border, but not in basolateral membrane. Moreover, an electro-chemical Na+ gradient, oriented with the lower potential on the inside, supported accumulation of the non-electrolytes above medium concentration only in the brush border membrane. L-Valine and D-glucose transport also were saturated at lower concentrations in brush border (10-20 mM) than in basolateral plasma membranes (30-50 mM). A third difference between the two membranes was found in the effectiveness of known inhibitors of D-glucose transport. In brush border membranes phlorizin was more potent than phloretin and 2', 3', 4'-trihydroxy-4-methoxy chalcone and cytochalasin B did not inhibit at all. In contrast, with the basolateral plasma membranes the order of potency was changed to phloretin = 2',3',4'-trihydroxy-4-methoxy chalcone greater than cytochalasin B greater than phlorizin. These results indicate the presence of different types of transport systems for monosaccharides and neutral amino acids in the luminal and contra-luminal region of the plasma membrane. Active transepithelial transport can be explained on the basis of the different properties of the non-electrolyte transport systems in the two cellular regions and an electro-chemical Na+ gradient that is dependent on cellular metabolism.

Animals↗
PubMed1976

Sugar and amino acid transport in animal cells.

The molecular basis of intracellular metabolism of nutrients and its control is quite well understood in animal cells. Comparable knowledge about solute entry into cells is still lacking, as, in contrast to metabolism, no chemical reactions seem to be directly associated with the known nutrient transport. Nevertheless, translocations of sugars and amino acids across the plasma membrane are specific and controlled processes, biologically as well as chemically. Recent advances in techniques for isolation of plasma membranes have made it feasible to study transport properties of animal cells without the complications encoutered in viable cells. This approach has been applied to sugar and amino acid transport in plasma membranes of several tissues, and intact transport systems for D-glucose, D-fructose, neutral L-amino acids, and dipeptides have been demonstrated. This demonstration of intact transport systems in an in vitro setting accomplishes the first step in the direction of molecular isolation of transport systems. Furthermore, the information obtained about the transport mechanism catalyzed by some systems has settled controversies on active nutrient transport. For example, electrogenic cotransport of sodium and D-glucose or of sodium and neutral L-amino acids has been shown to form the basis for active, sodium-dependent absorption of these nutrients. A consequence of this type of mechanism is interaction between sugar and amino acid transport via the common charged cosubstrate sodium. Moreover, different types of transport systems for the same substrate have been demonstrated in the luminal and contraluminal regions of the plasma membrane of epithelial cells, which explains unidirectional transepithelial transport. The luminal membrane contains sodium-dependent, active transport systems, and the contraluminal membrane passive, facilitated diffusion systems. In vivo, the lower intracellular sodium potential would result in concentrative nutrient uptake from the lumen, but would not influence exit on the contraluminal side. Variations in the electrical components of the sodium potential, which have not been measured, may explain apparently contradicting results on active sugar and amino acid transport with various tissue preparations.

Amino Acids↗
PubMed1975

INtestinal sugar transport: studies with isolated plasma membranes.

The luminal and the contraluminal plasma membranes of small intestinal epithelial cells were isolated separately and in the form of membrane vesciles. In the luminal membrane distinct transport systems exist for D-fructose and for D-glucose and glucalogs as evidenced by stereospecificity for the substrate, counter- and cotransport phenomena, and inhibitor specificity. D-Glucose is taken up against a concentration gradient when energy is supplied in the form of an electrochemical Na+ gradient between medium and intravesicular membrane space. The contraluminal membrane also possesses a glucose transport system. However, it is sensitive to different inhibitors than the glucose transport in the luminal membrane. D-Glucose uptake by the contraluminal membrane is not enhanced by Na+ gradients and can be characterized as "facilitated diffusion." The studies with the isolated plasma membranes allow a description of transcellular D-glucose transport in terms of the translocation events across single membranes.

Animals↗
PubMed1975

On the mechanism of sugar and amino acid interaction in intestinal transport.

The influence of amino acids on D-glucose transport was studied in isolated vesicles of brush border membrane from rat small intestine. It is demonstrated that: (a) Uptake of D-glucose by the membranes is inhibited by simultaneous flow of L- and D-alanine into the vesicles. (b) Addition of L-alanine to membranes pre-equilibrated with D-glucose causes efflux of this sugar. (c) The influence of amino acids on D-glucose is dependent on the presence of Na+. (d) The ionophorous agents monactin and valinomycin are able to prevent the transport interaction of D-glucose and amino acids. Monactin is effective in the presence of Na+ without further addition of other cations, while valinomycin is effective only with added K+, in accordance with the known specificity of these antibiotics. (e) The inhibitory effect increases with L-alanine concentration up to about 50 mM after which it levels off. The experiments provide evident that the Na+-dependent sugar and amino acid fluxes across the brush border membrane are coupled electrically.

Alanine↗
PubMed1975

Active alanine transport in isolated brush border membranes.

Uptake of L-alanine against a concentration gradient has been shown to occur with isolated brush border membranes from rat small intestine. An alanine transport system, displaying the following characteristics, was shown: (a) L-alanine was taken up and released faster than D-alanine; (b) Na+ as well as Li+ stimulated the uptake of both stereoisomers; (c) the uptake of L- and D-alanine showed saturation kinetics; (d) countertransport of L-alanine was shown; (e) other neutral amino acids inhibited L-alanine but not D-alanine entry when an electrochemical Na+ gradient across the membrane was present initially during incubation. No inhibition occurred in the absence of a Na+ gradient. The electrogenicity of L-alanine transport was established by three types of experiments: (a) Gradients of Na+ salts across the vesicle membrane (medium concentration greater than intravesicular concentration) supported a transient uptake of L-alanine above equilibrium level, and the lipophilic anion SCN- was the most effective counterion. (b) A gradient of K= across the membrane (vesicle greater than medium) likewise supported active transport of L-alanine into the vesicles provided the K= conductance of the membrane was increased with valinomycin. (c) Similarly, a proton gradient (vesicle greater than medium) in the presence of carbonyl cyanide p-trifluoromethoxyphenylhydrazone, an agent known to increase the proton conductance of membranes, produced an overshooting L-alanine uptake. A consideration of the possible forces, existing under the experimental conditions, suggests that the gradients of SCN-, K+ in the presence of valinomycin, and H+ in the presence of carbonyl cyanide p-trifluoromethoxyphenylhydrazone contribute to the driving force for L-alanine transport by creating a diffusion potential. Since the presence of Na+ was required in all experiments with active L-alanine transport these results support the existence of a transport system in the brush border membrane which catalyzes the co-transport of Na+ and L-alanine across this membrane.

Alanine↗
PubMed1975

Diabetes mellitus: changes in the transport properties of isolated intestinal microvillous membranes.

Isolated, small intestinal microvillous membranes from normal and acutely diabetic rats were compared with respect to D-glucose transport. D-Glucose was accumulated to a greater extent by diabetic membrane vesicles when supplied with energy in the form of a NaC1 or a NaSCN gradient across the vesicle membrane. The difference appeared to be caused by an ability of the diabetic membranes to maintain a higher driving force for active D-glucose transport and not by changes in the glucose "carrier." Increasing the glucose-independent Na-+-conductance of the membrane with monactin or gramicidin D reduced the active accumulation of D-glucose by membrane preparations from both control and diabetic groups. Concentrations of monactin and gramicidin D in the incubation medium of membrane vesicles from diabetic animals could be adjusted so that their D-glucose transport became indistinguishable from that of membranes from normal animals not treated with ionophores. These observatins suggest the microvillous membranes as one site where changes occur in acute diabetes. In addition, the change in the transport properties of the isolated membranes offer a rational explanation for the simultaneous elevation of active intestinal sugar, amino acid, and bile salt transport observed for intact intestinal tissue.

Animals↗
PubMed1974

Demonstration of electrogenic Na+-dependent D-glucose transport in intestinal brush border membranes.

Na(+)-coupled D-glucose transport was studied in isolated membrane vesicles from intestinal brush borders. Concentration gradients of SCN(-), K(+), and H(+) were established between the intravesicular solution and the incubation medium and their influence on D-glucose uptake from the medium was measured. A gradient (medium > vesicle) of NaSCN, but not of KSCN, produced a transient overshoot of D-glucose uptake above the equilibrium level. Similarly, an increase of the membrane conductance with valinomycin (K(+)-conductance) or with uncoupling agents of oxidative phosphorylation (H(+)-conductance) induced an overshooting D-glucose uptake, provided a (vesicle > medium) K(+)-gradient or a H(+)-gradient, respectively, was present in each case. The transient overshoot is evidence that D-glucose was taken up against its concentration gradient (up to 10-fold). The gradients of SCN(-), K(+) (in the presence of valinomycin), and H(+) (in the presence of uncouplers) are thought to contribute to the "driving" force for this "active" D-glucose transport by changing the electrical potential across the vesicle membrane and thus making the inside more negative (with respect to the medium). These experiments, therefore, provide evidence that the Na(+)-coupled D-glucose translocation across the brush border membrane is an electrogenic process, i.e., the positive charge associated with Na(+) is not compensated by the co-movement of an anion or the counter-movement of a cation via the glucose "carrier". The results imply that an electrical potential across the brush border membrane may play an important role in determining the transport of D-glucose by intact cells.

Animals↗